Connecting to a SCADA system

[ICON]

I am trying to connect my PLC to a SCADA using Modbus and the 485 port. I have written a custom script for the PLC to define the speed, bit length, and parity of the port as well as defiend the drop number of the PLC. However, when I try to poll the PLC from the SCADA I do not get a signal back. What I want to know is if the Modbus protcol is native, imbedded, or do I have to write a write a script to output the data. Note: If I need to write a script please provide me with an example of how to send and receive the information contained within the DM registries.

Thanks-

[support]

Which PLC model are you using? Only T100MD and T100MX support MODBUS on their COMM1 and COMM3 port. All you need to do is to define the baud rate on the SCADA program to the default value of 38,400, 8 data bit, 1 stop bit and no parity, no handshaking and define the device ID to be the same as what you have defined for the PLC (default is 01), then it should work transparently. Even if there is no program in the PLC the SCADA will be able to work with the PLC's using the MODBUS protocol. ?In fact, your PLC program should not write anything out of the serial port otherwise it will interfere with the SCADA polling of the PLC slave and results in no communication. Normally there is no need to change the PLC's baud rate (using SETBAUD) unless your SCADA is unable to use the PLC's default baud rate.

The easiest way to test the communication is to read from MODBUS register 40001 (binary address 0), which maps to input 1-16, then turn ON any of the first 16 inputs on the PLC and observe the changes to the value of 40001.

you are using the bit mapping instead of word mapping, please download the following updated document for the MODBUS mapping to the PLC's register:

http://www.tri-plc.com/appnotes/modbus-map.pdf